Integrated circuit cell library

ABSTRACT

According to the invention, an ASIC cell library for use in creation of custom integrated circuits is disclosed. The ASIC cell library includes some first cells and some second cells. Each of the second cells includes two or more kernel cells. The ASIC cell library is at least 5% comprised of second cells. In various embodiments, the ASIC cell library could be 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more comprised of second cells.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a nonprovisional of and claims priority toU.S. Prov. Pat. Appl. No. 60/298,832, entitled “MULTIPLEXOR-BASEDDIGITAL DESIGN,” filed Jun. 15, 2001 by Sterling R. Whitaker et al, theentire disclosure of which is herein incorporated by reference for allpurposes.

[0002] This application is also related to the following commonlyassigned, concurrently filed U.S. patent applications, each of which isalso incorporated herein by reference in its entirety for all purposes:U.S. Pat. Appl. Ser. No. --/---,---, entitled “DIGITAL DESIGN USINGSELECTION OPERATIONS,” by Sterling R. Whitaker, Lowell H. Miles, andEric G. Cameron (Attorney Docket No. 021145-001600US); U.S. Pat. Appl.Ser. No. --/---,---, entitled “PASS-TRANSISTOR VERY LARGE SCALEINTEGRATION,” by Gary K. Maki and Prakash R. Bhatia (Attorney Docket No.021145-001700US); U.S. Pat. Appl. No. --/---,---, entitled “OPTIMIZATIONOF DIGITAL DESIGNS,” by Sterling R. Whitaker and Lowell H. Miles(Attorney Docket No. 021145-001800US); U.S. Pat. Appl. Ser. No.--/---,---, entitled “DIGITAL LOGIC OPTIMIZATION USING SELECTIONOPERATIONS,” by Sterling R. Whitaker, Lowell H. Miles, Eric G. Cameron,and Jody W. Gambles (Attorney Docket No. 021145-002000US); and U.S. Pat.Appl. Ser. No. --/---,---, entitled “DIGITAL CIRCUITS USING UNIVERSALLOGIC GATES,” by Sterling R. Whitaker, Lowell H. Miles, Eric G. Cameron,Gregory W. Donohoe, and Jody W. Gambles (Attorney Docket No.021145-002100US). These applications may be referred to herein as “theUniversal-Logic-Gate applications.”

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

[0003] The U.S. Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms of GrantNo. NAGS-9152 awarded by NASA.

BACKGROUND

[0004] This invention relates in general to digital circuits and, morespecifically, to design of digital circuits that are laid-out withcells.

[0005] Mathematics is one attempt for humankind to understand theuniverse around them. As technological advancement occurs, mathematicalconcepts and algorithms grow to enable and/or support thoseadvancements. Within the context of digital design, Boolean logic is themathematical construct used to manipulate and optimize digital circuits.Nearly every electronic device today relies upon some type of Booleanlogic for any embedded digital circuits. Other mathematical constructs,however, are possible that allow further optimization of digitaldesigns. Changes to the processing of digital design are necessary whenavoiding Boolean logic elements.

[0006] Today application specific integrated circuit (ASIC) arespecified using netlists of library cells for a particular process of afoundry or fabrication facility. These netlists are used to fabricateintegrated circuits made up of the library cells. A few hundred librarycells are typically available for a particular process that include ANDgates, OR gates, flip-flops (F/F), and buffers. When a new fabricationprocess is developed, engineers custom layout each of the library cellsto get the most optimal performance from each cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention is described in conjunction with theappended figures:

[0008]FIG. 1A is a block diagram of an embodiment of a basic cellcomposed of kernel cells;

[0009]FIG. 1B is a block diagram of another embodiment of a basic cellcomposed of a memory and a buffer kernel cells;

[0010]FIG. 1C is a block diagram of yet another embodiment of a basiccell composed of a selection and memory kernel cells;

[0011]FIG. 1D is a block diagram of still another embodiment of a basiccell composed of a selection and a buffer kernel cells;

[0012]FIG. 2A is a block diagram of an embodiment of a memory cell witha synchronous reset;

[0013]FIG. 2B is a block diagram of another embodiment of a memory cellwith an asynchronous reset;

[0014]FIG. 3 is a block diagram of an embodiment of a universal logicgate layout;

[0015]FIG. 4 is a block diagram of an embodiment of a memory kernel celllayout;

[0016]FIG. 5 is a block diagram of an embodiment of a buffer kernel celllayout;

[0017]FIG. 6 is a block diagram of an embodiment of a basic cell abuttedtogether from the kernel cells of FIGS. 3-5;

[0018]FIG. 7 is a block diagram of an embodiment of two basic cells laidout together;

[0019]FIG. 8A provides a schematic illustration of an enhancement-modetransistor;

[0020]FIG. 8B provides a schematic illustration of a depletion-modetransistor;

[0021]FIG. 8C provides a circuit layout for a universal logic gateaccording to an embodiment of the invention that uses depletion-modetransistors;

[0022]FIG. 9A is a flow diagram illustrating how inversions may beremoved in logical expressions implemented in embodiments of theinvention;

[0023]FIG. 9B is a flow diagram illustrating how nodes may be reduced inlogical expressions implemented in embodiments of the invention;

[0024]FIG. 9C is a flow diagram illustrating how nodes may be combinedin logical expressions implemented in embodiments of the invention;

[0025]FIG. 9D is a flow diagram illustrating how set and reset inputsmay be used in performing optimizations according to embodiments of theinvention;

[0026]FIG. 10 is a flow diagram illustrating an embodiment for a ULGnetlist optimization;

[0027]FIG. 11 provides a schematic illustration of a computer system onwhich methods of the invention may be embodied;

[0028]FIG. 12 is a flow diagram of an embodiment of a process forpreparing a ULG ASIC cell library;

[0029]FIG. 13 is a block diagram of an embodiment of a design flow thatuses syntactic manipulation after synthesis;

[0030]FIG. 14 is a block diagram of another embodiment of a design flowthat uses syntactic manipulation and the ULG ASIC cell library;

[0031]FIG. 15 is a block diagram of another embodiment of a design flowthat uses the ULG ASIC cell library for the final netlist;

[0032]FIG. 16 is a block diagram of yet another embodiment of a designflow that combines synthesis and syntactic manipulation into a singletool;

[0033]FIG. 17 is a block diagram of still another embodiment of a designflow that uses a verification tool throughout the design flow;

[0034]FIG. 18 is a block diagram of still another embodiment of a designflow that uses a verification tool throughout the design flow and afterfabrication; and

[0035]FIG. 19 is a flow diagram of an embodiment of a design process.

[0036] In the appended figures, similar components and/or features mayhave the same reference label. Further, various components of the sametype may be distinguished by following the reference label by a dash anda second label that distinguishes among the similar components. If onlythe first reference label is used in the specification, the descriptionis applicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

[0037] The ensuing description provides preferred exemplaryembodiment(s) only, and is not intended to limit the scope,applicability or configuration of the invention. Rather, the ensuingdescription of the preferred exemplary embodiment(s) will provide thoseskilled in the art with an enabling description for implementing apreferred exemplary embodiment of the invention. It is to be understoodthat various changes may be made in the function and arrangement ofelements without departing from the spirit and scope of the invention asset forth in the appended claims.

[0038] In one embodiment, the present invention provides an ASIC celllibrary for use in creation of custom integrated circuits. The ASIC celllibrary includes some first cells and some second cells. Each of thesecond cells includes two or more kernel cells. The ASIC cell library isat least 5% comprised of second cells. In various embodiments, the ASICcell library could be 10% or more, 20% or more, 30% or more, 40% ormore, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more,or 95% or more comprised of second cells. In various embodiments, atotal number of kernel cells could be less 10, 20, 30, 40, 50, 75, or100.

[0039] In another embodiment, the present invention provides anintegrated circuit cell library for use in circuits having digitallogic. A first percentage of the integrated circuit cell library arefirst cells and a second percentage of the integrated circuit celllibrary are second cells. Each of the second cells includes a selectioncircuit. The second percentage is more than 5% of the integrated circuitcell library.

[0040] In yet another embodiment, the present invention provides anintegrated circuit cell library for use in circuits having digitallogic. The integrated circuit cell library includes first cells andsecond cells. The first cells are a first percentage of the integratedcircuit cell library and the second cells are a second percentage of theintegrated circuit cell library. Kernel cells are used in forming thesecond cells. At least some of the second cells include at least twokernel cells coupled together through abutment.

[0041] I. Cells

[0042] In one embodiment, a basic cell is a construct that includes oneor more of a universal logic gate (ULG), a memory element or flip-flop(F/F), and/or a buffer. In this embodiment, the ULG is a multiplexor orselect circuit. The ULG, memory element, and buffer are implemented witha relatively-small number of kernel cells, which typically have layoutsthat are individually optimized, and often, by hand. The kernel cellsare arranged into the higher-level basic cells having at least one ofthe ULG, memory element, and buffer, but the basic cells do not havemore than one of any type of kernel cell in this embodiment. For a givensemiconductor process, there is a ULG ASIC cell library which iscomposed of the basic cells and specialized cells. These specializedcells may differ from the basic cell construct and could include, forexample, clock dividers, memory arrays, analog circuits, phase-lockedloops, oscillators, analog circuits, etc.

[0043] Referring first to FIG. 1A, an embodiment of a basic cell 100 isshown in block diagram form. This embodiment includes all three of a ULGor selection circuit 104, a memory cell 108 and a buffer 112. Some ofthe kernel cell components of the basic cell 100 are shown in ageneralized manner. The ULG 104 is shown having any number of data andselection control inputs, however the relationship between the maximumdata inputs for a number of selection control inputs follows thefollowing relationship 2^(y)=I. The memory kernel cell 108 shown is aresetable D F/F. A buffer kernel cell 112 shown has both an invertingand non-inverting output, although, other buffer implementations willhave either an inverting or non-inverting output.

[0044] The ULG 104 in this embodiment is implemented with a multiplexor.Multiplexors can be used to implement any Boolean function, but are notBoolean operators. Combinatorial logic in conventional designs is notimplemented with selection functions, but uses Boolean logic gates.Further, multiplexors in conventional circuits are converted to Booleanequivalents during synthesis.

[0045] The below Table I shows the fourteen kernel cells used in oneembodiment: TABLE I ULG Component Symbol Description ULG U 8 to 1 (U8),4 to 1 (U4) or 2 to 1 (U2) Multiplexors Memory Cells D D F/F (D1) DRResetable D F/F - Synchronous (DR1), Clock Edge Synchronization (DR2) orAsynchronous (DR3) DS Setable D F/F - Synchronous (DS1), Clock EdgeSynchronization (DS2) or Asynchronous (DS3) Buffers B Non-invertingbuffer (B1) or Hybrid Inverting and Non-inverting buffer (B2) BNInverting buffer (BN1) CB High-drive buffer (CB1) ZB Tristatable buffer(ZB1)

[0046] The embodiment of the kernel cells in the above Table I could beaugmented in other embodiments to include other cells. The ULGs couldinclude multiplexors of any size, for example, 16 to 1, 32 to 1, 64 to1, etc. Larger multiplexors could be formed with a number of smallermultiplexors if a larger multiplexor is not supported in the kernelcells. Various other types of memory cells could also be supported suchas EEPROM, EPROM, PROM, DRAM, SRAM, NVRAM, magnetic core memory, J-KF/Fs, setable and resetable F/Fs, various F/F with scan ATPG capability,etc. The J-K, setable, or resetable functionality of a F/F can beimplemented by a D F/F and logic that can be embedded in the mux beforeor after the D F/F. The F/Fs could also be falling edge triggered insome embodiments. Also the buffers could be of various strengths andsizes. Some buffers could support input and output pins of the chip withvarious thresholds, voltages, etc.

[0047] Table II lists the various configurations in which kernel cellsare used to create basic cells 100-1 that use all of a ULG 104, a memorycell 108 and a buffer cell 112 such as the example in FIG. 1A. Thesebasic cells 100-2 are the variations found in one embodiment of the ULGASIC cell library. TABLE II Configuration Basic Cell Mux → Mem → TypeBuf Various Basic Cell Layout Names UDB U → D → B U2D1B1, U4D1B1,U8D1B1, U2D1B2, U4D1B2, U8D1B2, UDBN U → D → BN U2D1BN1, U4D1BN1,U8D1BN1 UDZB U → D → ZB U2D1ZB1, U4D1ZB1, U8D1ZB1 UDRB U → DR → BU2DR1B1, U2DR2B1, U2DR3B1, U2DR1B2, U2DR2B2, U2DR3B2, U4DR1B1, U4DR2B1,U4DR3B1, U4DR1B2, U4DR2B2, U4DR3B2, U8DR1B1, U8DR2B1, U8DR3B1, U8DR1B2,U8DR2B2, U8DR3B2 UDRBN U → DR → BN U2DR1BN1, U2DR2BN1, U2DR3BN1,U4DR1BN1, U4DR2BN1, U4DR3BN1, U8DR1BN1, U8DR2BN1, U8DR3BN1 UDRZB U → DR→ ZB U2DR1ZB1, U2DR2ZB1, U2DR3ZB1, U4DR1ZB1, U4DR2ZB1, U4DR3ZB1,U8DR1ZB1, U8DR2ZB1, U8DR3ZB1 UDSB U → DS → B U2DS1B1, U2DS2B1, U2DS3B1,U2DS1B2, U2DS2B2, U2DS3B2, U4DS1B1, U4DS2B1, U4DS3B1, U4DS1B2, U4DS2B2,U4DS3B2, U8DS1B1, U8DS2B1, U8DS3B1, U8DS1B2, U8DS2B2, U8DS3B2 UDSBN U →DS → BN U2DS1BN1, U2DS2BN1, U2DS3BN1, U4DS1BN1, U4DS2BN1, U4DS3BN1,U8DS1BN1, U8DS2BN1, USDS3BN1 UDSZB U → DS → ZB U2DS1ZB1, U2DS2ZB1,U2DS3ZB1, U4DS1ZB1, U4DS2ZB1, U4DS3ZB1, U8DS1ZB1, U8DS2ZB1, U8DS3ZB1

[0048] With reference to FIG. 1B, a block diagram of another embodimentof a basic cell 100-2 composed of memory and buffer kernel cells 108,112 is shown. This is but one example of a basic cell 100-2 of thisgeneral configuration. Other basic cells of this general configurationthat could be found in an embodiment of a ULG ASIC cell library arelisted in Table III. TABLE III Basic Cell Configuration Type Memory →Buffer Various Basic Cell Layout Names DB D → B D1B1, D1B2 DBN D → BND1BN1 DZB D → ZB D1ZB1 DRB DR → B DR1B1, DR2B1, DR3B1, DR1B2, DR2B2,DR3B2 DRBN DR → BN DR1BN1, DR2BN1, DR3BN1 DRZB DR → ZB DR1ZB1, DR2ZB1,DR3ZB1 DSB DS → B DS1B1, DS2B1, DS3B1, DS1B2, DS2B2, DS3B2 DSBN DS → BNDS1BN1, DS2BN1, DS3BN1 DSZB DS → ZB DS1ZB1, DS2ZB1, DS3ZB1

[0049] Referring next to FIG. 1C, a block diagram of yet anotherembodiment of a basic cell 100-3 is shown that is composed of ULG andmemory kernel cells 104, 108. This is just one example of the varioussimilar basic cells 100-3 that might form an embodiment of the ULG ASICcell library. Other possible configurations are enumerated in Table IVbelow. TABLE IV Basic Cell Configuration Type Mux → Mem Various BasicCell Layout Names UD U → D U2D1, U4D1, U8D1 UDR U → DR U2DR1, U4DR1,U8DR1, U2DR1, U4DR2, U8DR2 U2DR3, U4DR3, U8DR3 UDS U → DS U2DS1, U4DS1,U8DS1, U2DS2, U4DS2, U8DS2, U2DS3, U4DS3, U8DS3

[0050] With reference to FIG. 1D, a block diagram of still anotherembodiment of a basic cell 100-4 composed of ULG and buffer kernel cells104, 112 is shown. There are other possible configurations of this typeof basic cell 100-4. The variations of this basic cell 100-4 for oneembodiment of the ASIC library are listed in Table V. From Tables II-V,around 80% of the 142 available basic cells include ULG circuits. The142 basic cells are based upon the 14 kernel cells of Table I.

[0051] Although the embodiment in Tables II-V show some possible basiccells, other embodiments could include additional basic cells. Theseadditional basic cells could be optimized for output power, powerconsumption, layout area, response time, leakage, etc. such that thereare multiple cells with the same logical properties, but that areoptimized for particular circumstances. For example, there may be threenon-inverting buffers of having different drives to support largerfanout and/or higher speeds. In various embodiments, there could be lessthan, for example, 100, 75, 50, 40, 30, 20, or 10 kernel cells. At thelower limit, there is three kernel cells in one embodiment with just oneof each of the types of kernel cells. TABLE V Basic Cell ConfigurationType Mux → Buf Various Basic Cell Layout Names UB U → B U2B1, U4B1,U8B1, U2B2, U4B2, U8B2 UBN U → BN U2BN1, U4BN1, U8BN1

[0052] The building blocks of a digital circuit could be abstractedbeyond the ULG ASIC cell library. In some embodiments, the ULG ASIC celllibrary components could be combined in higher-level macro cells such asadders, multipliers, registers, barrel shifters, ALUs, comparators,decoders, state machines, counters, etc. There could be thousands ofpossible macro cells. Further, designs can be abstracted to a levelhigher than the macro cells by using cores that implement higher levelfunctions such as microprocessors, graphics processors, interface bussesor ports, digital signal processors, etc. These cores could use macrocells and/or components from the ULG ASIC cell library. Often the coresare written in a hardware description language (HDL) that can be easilysynthesized into any ULG ASIC cell library for a particular process.

[0053] With reference to FIGS. 2A and 2B, various embodiments of amemory kernel cell 108 are shown in block diagram form. Theseembodiments divide the D F/F 208 out from the memory cell and implementsome functionality with a separate buffer cell 204. In variousembodiments, the buffer cell 204 could be used to customize the D F/F208 with synchronous reset of FIG. 2A or asynchronous reset of FIG. 2B.In other embodiments, a separate circuit could be used to make a D F/F208 behave as a setable D F/F, a J-K F/F or a F/F with scan capability.In other embodiments, the separate circuit could be implemented with aselection circuit.

[0054] This buffer cell 204 in an ASIC cell library could be used forother purposes also. For example, an 8 to 1 mux function could beimplemented with a buffer cell 204 and a 4 to 1 mux 104 in somecircumstances to reduce the chip area needed to implement thefunctionality. Table VI shows the thirteeen kernel cells used in thisembodiment. Table VII shows a truth table for the enable buffer 204where the enable input is R, the input is D and the output is Q. TABLEVI Kernel Cell Component Symbol Description ULGs U 8 to 1 (U8), 4 to 1(U4) or 2 to 1 (U2) Multiplexors Memory Cells D D F/F (D1) DS Setable DF/F - Synchronous (DS1), Clock Edge Synchronization (DS2) orAsynchronous (DS3) Buffers B Non-inverting buffer (B1) or HybridInverting and Non-inverting buffer (B2) BN Inverting buffer (BN1) EBNInverting buffer with an enable input (EBN1) CB High-drive buffer (CB1)ZB Tristatable buffer (ZB1)

[0055] TABLE VII R D Q 0 0 1 0 1 1 1 0 1 1 1 0

[0056] II. Layout of Cells

[0057] Each fabrication process at a fab or foundry generally has aconventional ASIC cell library that is customized for that process. Eachof the hundreds of cells in the conventional ASIC cell library istypically manually laid out to optimize its configuration. In thisembodiment, however, a small number of customized kernel cells are usedto automatically or manually compile the basic cells 100. For a targetfabrication process, care is taken to optimize the layout of kernelcells 104, 108, 112 for factors such as power consumption, chip area,number of masks, number of process steps, yield, capacitance,inductance, resistance, glitches, I/O placement, etc. In some cases, thefabrication processes are similar enough to other fabrications processesthat only minor tweaking to kernel cells is done.

[0058] With reference to FIG. 3 a block diagram of an embodiment of aULG layout 300 is shown. A cloud graphic is used to represent the layoutof the circuit to implement the 2 to 1 ULG 104. Input ports 316, 320 andI/O 324 are detailed within the circuit cloud, more specifically, portsfor the inputs (I₀ and I₁) 316 and the select (Y₀) 320 and a trace isshown for the Q output 324. For the ULG kernel cell, the input andselect signals are ports 316, 320 within the ULG layout 300. The ULGcircuit 304 is attached by traces to latitudinal power traces 308, 312.

[0059] Abutment is used to link certain signals by coupling adjacentkernel cells. Adjacent placement of the cells may join the abutted I/Oor a small conductive trace may join the abutted I/O. The kernel cellshave a uniform height and differing depths such that the power traces308, 312 for each kernel cell align with the next kernel cell. Also,certain I/O signals use a uniform latitude. For example the Q output 324of the ULG layout 304 would align latitudinally with an input for anadjacent memory or buffer kernel cell.

[0060] Referring next to FIG. 4, a block diagram of an embodiment of amemory kernel cell layout 400 is shown. This memory circuit 404implements a D F/F with a D input 412, a clock input 408 and a Q output416. Coupled to the memory circuit 404 are a V_(DD) and V_(SS) powerbusses 308, 312. The height of the memory kernel cell layout 400 is thesame as the ULG cell layout 300 such that the power busses for bothkernel cells align latitudinally.

[0061] With reference to FIG. 5, a block diagram of an embodiment of abuffer kernel cell layout 500 is shown. As with the other kernel cellcircuits 304, 404, a non-inverting buffer kernel cell circuit 504 iscoupled to power busses 308, 312 with a height uniform to the otherkernel circuits 304, 404. The buffer circuit includes a D input 508 anda Q output 512, where the D input 508 is latitudinally aligned with theoutputs from either a ULG circuit 304 or a memory circuit 404. In thisembodiment, the Q output 512 is offset from the latitude of the D input508.

[0062] Referring next to FIG. 6, a block diagram of an embodiment of abasic cell 600 abutted together from three kernel cells 300, 400, 500 isshown. A U2 ULG, D1 F/F and B1 non-inverting buffer kernel cells 300,400, 500 are connected in serial to form the basic cell 600. The powerbusses 308, 312 for each kernel cell 300, 400, 500 align to form alarger whole. The Q output 324 from the U2 ULG circuit 304 aligns withthe D input 412 to the D1 memory circuit 404, and the Q output 416 fromthe D1 memory circuit 404 aligns with the D input 508 to the B1 buffercircuit 504. Other embodiments could have additional power busses, forexample, a substrate bus connection.

[0063] With reference to FIG. 7, a block diagram of an embodiment of twobasic cells 600, 704 laid out together in a row 700 is shown. Duringlayout of a chip, all the ULG ASIC cells are arranged. The basic cells600, 704 are aligned in horizontal rows. In some cases (not depicted),there is routing of one or more signals between the basic cells 600,704. In the depicted embodiment, an output from a first basic cell 704is coupled with a trace 712 to an input of a second basic cell 600. Theclock inputs for both basic cells 600, 704 are latitudinally alignedsuch that a clock bus can pass strait across a row 700 of basic cells.

[0064] In some embodiments, additional size reductions are realized inthe ULGs by having them comprise one or more depletion-mode transistors.Schematic diagrams are provided in FIGS. 8A and 8B that compareenhancement-mode and depletion-mode transistors. FIG. 8A shows thestructure of an n-type enhancement-mode transistor 802 that comprises asource 804, a drain 806, and a gate 810. Connections are made with thesource 804 and drain 806 respectively through pads 812 and 814. The gate810 usually comprises a metal formed over an oxide such as SiO₂. Forsuch an n-type transistor 802, both the source 804 and drain 806comprise n-doped regions in ap-doped substrate. The transistor operatesso that when at least a threshold voltage is applied to the gate 810,current flows between the source 804 and drain 806 through anintermediate channel region. In circuits, the enhancement-modetransistor 802 is denoted with symbol 820.

[0065] The depletion-mode transistor 842 illustrated in FIG. 8B alsocomprises a source 844, a drain 846, and a gate 850 formed over an oxide848, with connections to the source 844 and drain 846 providedrespectively by pads 852 and 854. For the depletion-mode transistor,however, the channel region 856 between the source 844 and drain 846 isalso n-doped, allowing the flow of current even without a gate voltage.The current can be stopped by applying at least a negative cutoffvoltage to the gate 850. In circuits, the depletion-mode transistor 842is denoted with symbol 860.

[0066]FIG. 8C provides an example of a circuit for a ULG element thatexploits the different properties of enhancement- and depletion-modetransistors to allow a smaller circuit area than a ULG design that usesonly enhancement-mode transistors. The illustration is provided for theU8 cell, which acts as an 8:1 multiplexor. The U8 cell 872 compriseseight inputs 874 labeled I_(0 . . . 8), three selection controls 876labeled Y[0 . . . 2], and one output 878 labeled Q. The multiplexingfunctions of the cell are governed by the action of 48 transistors, ofwhich half are provided as depletion-mode transistors. Each of thecontrols 876 and its inversion is provided to a transistor along thepath of one of the inputs 874, with the inversions being effected byinverters 880. To effect the multiplexing functions, the depletion-modetransistors are distributed according to the level of the control. Forthe least significant control, the depletion-mode transistors arepositioned alternately; for the next significant control, they arepositioned alternately in pairs; for the next significant control, theyare positioned alternately in quads; etc. Thus, for the U8 cell 872,depletion-mode transistors for Y[0] are provided for inputs I₀, I₂, I₄,and I₆, and for the inverted control YN[0], depletion-mode transistorsare provided for inputs I₁, I₃, I₅, and I₇. For Y[1], depletion-modetransistors are provided for inputs I₀, I₁, I₄, and I₅, and for theinverted control YN[1], they are provided for inputs I₂, I₃, I₆, and I₇.Similarly, depletion-mode transistors are provided for Y[2] for inputsI₀, I₁, I₂, and I₃, and for YN[2] for inputs I₄, I₅, I₆, and I₇.

[0067] For certain embodiments of the ULGs that comprise depletion-modetransistors, this pattern may be used for a cell of any size. For a U2^(n+1) ULG that has 2^(n+1) inputs and n+1 controls, depletion-modetransistors may be provided for each control/input combination asfollows:

[0068] (i) For control Y[i=0 . . . n], depletion-mode transistors areprovided for inputs I_(k) where k<2^(i) mod 2^(i+1);

[0069] (ii) For control YN[i=0 . . . n], depletion-mode transistors areprovided for inputs I_(k), where k′>2^(i) mod 2^(i+1).

[0070] In other embodiments, a different distribution of depletion-modetransistors may be used to implement the multiplexing functions of theULG.

[0071] III. Logical Structures

[0072] In addition to the structural characteristics described above,there are a number logical properties and features that may be used bothto characterize individual cells and to characterize libraries of suchcells. An example of a formalism that may be used in one embodiment todescribe the functionality of the basic cells and from which at leastsome such logical characterizations may be extracted is now described.

[0073] As discussed above, the selection circuits embodied by ULGs usedfor forming basic cells may be implemented using 2:1, 4:1, 8:1, orperhaps even larger, multiplexors. The inputs and control of eachmultiplexor are programmed to achieve the desired logicalcharacteristics of the cell. The following syntax has been developed todescribe the programming of such inputs and selection controls in ageneral fashion:

[0074] Q<QN>.xxxx<E>Y[n−1:0] I₂ _(^(n)) ⁻¹I₂ _(^(n)) ⁻² . . .I₀<R/S><CLK>(Parameter=Option);

[0075] In addition to describing the programming of the ULG kernelcells, the syntax may be used to describe any of the basic cells,including both those that comprise ULG kernel cells and those that donot comprise ULG kernel cells. Optional parameters in the syntax aredenoted with angular brackets and the components of the syntax aresummarized in Table I. TABLE VIII Component Meaning Q Non-invertingoutput <QN> Optional inverting output .xxxx Cell name <E> Optionaltri-state enable Y[n − 1:0] Control variables State variables I_(x)Truth-table state Next state <R/S> Optional reset or set input <CLK>Optional clock input (Parameter = Option) Selects an option ; Terminator

[0076] There are a number of features of the syntax worthy of comment.The first component of the syntax indicates the output of the cell,using either Q or QN respectively to denote the output Q or {overscore(Q)}. This is followed by the name of the cell, which is generallyconstructed by concatenating the names of the kernel cells comprised bythe cell. The names of the kernel cells have been set forth above inTable VIII. Thus, for example, a basic cell that comprises a D F/F and anon-inverting buffer would be named .DB (D→B); a basic cell thatcomprises a resetable D F/F and an inverting buffer would be named .DRBN(DR→BN); a basic cell that comprises a ULG multiplexor and a setable DF/F would be named .UDS (U→DS); and a basic cell that comprises a ULGmultiplexor, a D F/F, and a tristatable buffer would be named .UDZB(U→D→ZB). It is noted that some of these examples of basic cells includea ULG multiplexor kernel cell while others do not, but all of these maybe described with the syntax.

[0077] In those instances where the syntax is used to describe a basiccell comprising a ULG, the number of selection control inputs providedto the ULG is n. In a specific embodiment, the control inputs areordered by significance, with the most significant control on the leftand the least significant control on the right. While the syntax isequally robust for describing basic cells for any value of n, forpurposes of explanation the examples provided herein generallycorrespond to cases in which n<3. The states of the n control inputs Ydictate which of 2^(n) inputs are passed to the output of the ULG. Whilein some instances, the control inputs may be identified individually, inother instances a range of control inputs is identified by using a colonin the argument of Y. Specifically, “Y[a:b]” is intended to refer to thefull expression “Y[a] Y[a−1]Y[a−2 . . . Y[b+2]Y[b+1]Y[b].” The set I₂_(^(n)) ⁻¹I₂ _(^(n)) ⁻² . . . I₀ represents the logical function to beapplied by the cell, and as discussed in greater detail below maycomprise a truth table for implementing a combinational logic device ormay comprise an identification of the next state of a sequentialcircuit. In some embodiments, these logical states I₂ _(^(n)) ⁻¹I₂_(^(n)) ⁻² . . . I₀ may be assigned to logical 1's or 0's (sometimesreferred to herein as “base Boolean values”), but may more generallyinclude map-entered variables as well. For the basic cell, thiscorresponds to a connection to VDD for a logic 1, to a connection to VSSfor a logic 0, and to a connection to a signal for a mapped enteredvariable. Parameters such as the type of reset, i.e. asynchronous,synchronous, or clock-edge, are assigned and enclosed in parentheses atthe end of the statement.

[0078] This formalism permits the expression of a number ofmanipulations that are possible with embodiments of the invention andwhich are discussed in detail in order to enable one of skill in the artto perform such manipulations. The nature of such manipulations may beclarified with a simple example for the combinational logic functionC=A+{overscore (B)}. This logic function may be expressed in a concisehardware description language (“CHDL”) formalism as follows:

[0079] C .UB A B VDD VDD VSS VDD;

[0080] That this is a correct implementation of the logic function inwhich C is equal to “A or not B” is evident by comparing the entries inthe expression to the syntax discussed above. The name of the cell .UBindicates that the function is implemented with a cell that comprises auniversal logic gate U and a non-inverting buffer B. On either side ofthe name, the parameters involved in the function are denoted, with theleft-most component of the expression C indicating the output, and thevariables to the right of the name A and B indicating the inputs. Thefollowing four entries before the terminator define the following truthtable I₂ _(^(n)) ₁I₂ _(^(n)) ⁻² . . . I₀ for the combinational function,with VDD being equivalent to a logic 1 and VSS being equivalent to alogic 0. The individual truth-table states I are noted: TABLE IX C A BI₃= 1 0 0 I₂= 1 0 1 I₁= 0 1 0 I₀= 1 1 1

[0081] When logical operations are performed on expressions in thisformalism, they indicate directly how the resulting expression may beimplemented with basic blocks in accordance with an embodiment of theinvention. For example, a simple logical operation is inversion of theoutput, which may be implemented by using an inverting buffer:

[0082] CN .UBN A B VDD VDD VSS VDD;

[0083] As can be seen, the same truth table as that defined in Table IXis used for implementing {overscore (C)}, but the implementation is witha basic cell comprising a universal logic gate U and an inverting bufferBN. An alternative implementation of {overscore (C)} uses the same .UBbasic cell, but instead uses a different truth table by inverting all ofthe input states:

[0084] CN .UB A B VSS VSS VDD VSS;

[0085] In other instances, alternative implementations of the samelogical function may be achieved by performing operations on the controlinputs. For example, the control inputs A and B may be permuted.Permuting the control for the function acts to rearrange the truthtable. In an embodiment that includes this example, the truth-tablestates I₃ and I₀ remain in the same position because they representstates where both controls are high or both are low. States I2 and I1,which represent states where one control is high and the other is low,are interchanged:

[0086] C .UB B A VDD VSS VDD VDD;

[0087] A permutation of the truth table may also result from inversionof one or more of the control inputs. In this example, inverting theleast significant control B interchanges neighboring states in the truthtable:

[0088] C .UB A BN VDD VDD VDD VSS;

[0089] This alternative expression for may be viewed as defining animplementation for C that uses the general truth table for X+Y, but withcontrol inputs defined so that X=A and Y={overscore (B)}. If the nextsignificant control A is inverted in the original expression instead,neighboring pairs of states in the truth table are interchanged:

[0090] C .UB AN B VSS VDD VDD VDD;

[0091] It is evident that this expression implements the general truthtable for {overscore (X)}+{overscore (Y)}, but with control inputsdefined so that X={overscore (A)} and Y=B. If both control inputs areinverted,

[0092] C .UB AN BN VDD VSS VDD VDD;

[0093] The truth table in this expression implements the generalfunction {overscore (X)}+Y, but with control inputs defined so thatX={overscore (A)} and Y={overscore (B)}.

[0094] The CHDL syntax also permits control variables to be entered aselements in the truth-table states. For example, the syntax makes iteasy to recognize that the result C is high whenever A is high and thatC takes the value of BN when A is low. This may be expressed in thisCHDL syntax as

[0095] C .UB A VDD BN;

[0096] and corresponding to the truth table shown in Table X: TABLE X CA I₁= 1 0 I₀= {overscore (B)} 1

[0097] Equally, the syntax makes it easy to recognize that the result Cis high whenever B is low and that C takes the value of A when B is low.This may be expressed in this CHDL syntax as

[0098] C .UB B A VDD;

[0099] and corresponding to the truth table shown in Table XI: TABLE XIC B I₁= A 0 I₀= 1 1

[0100] Not only does the CHDL syntax presented here easily admit controlvariables to be presented as map-entered variables, but this sameability is manifested in the implementations with the cells describedabove. In particular, either of the two above examples may be as easilyimplemented using a combination of a ULG and buffer (“.UB”) as is anytruth table that uses the basic Boolean variables 0 and 1 exclusively.Implementation of all of these logical functions is simply a matter ofassigning the truth-table states and control variables in accordancewith the universal logic elements as described above.

[0101] The formalism thus makes clear that embodiments of the inventionpermit the implementation of a diverse range of logical functions.Specific examples of some of these properties are now discussed ingreater detail. In discussing logical properties that may be exploitedin certain embodiments of the invention, reference is sometimes made tothe formalism explained above. Such reference is made primarily forreasons of convenience and is not in any way intended to limit the scopeof the invention. In particular, it will be evident to those of skill inthe art that it is possible to implement each legitimate syntacticalexpression in the formalism with the cells discussed above. Accordingly,the logical properties of the formalism correspond directly to logicalfunctions that may be implemented with the cell arrangements indifferent embodiments.

[0102] One property of the formalism, and therefore also of the cellarrangements, is that no high-level distinction is made betweencombinational and sequential circuits; both such circuits are merelyspecial cases of the more general types of logical functions that may beimplemented. A combinational circuit is one in which the output(s) arepredetermined functions of the input(s). As such, the logic implementedby a combinational circuit can be represented by a truth table settingforth a mapping between all possible Boolean states of the input(s) tothe Boolean states of the output(s). This may be contrasted with asequential circuit in which the logical application of the circuitrelies on a history of past inputs. The application of such logic mayinstead be represented with a next-state equation that maps the pastinput(s) to the output(s). Embodiments of the invention described hereinare not restricted either to combinational or sequential logic. Forexample, only slight differences in cells are needed to implement thefollowing syntactic CHDL expressions:

[0103] Q .UB A B VDD VDD C VSS;

[0104] Q .UD A B VDD VDD C VSS CLK;

[0105] The first of these expressions represents a combinational logicfunction and the second represents a sequential logic function. In otherembodiments, the formalism and corresponding cell implementations mayinclude both combinational and sequential aspects so that acharacterization of the function is not properly limited to eithercategory. This additional flexibility permits certain optimizations,some of which are discussed below, that are not available when limitedto either combinational or sequential logic.

[0106] This additional flexibility also arises in part from the moregeneral character of cells made in accordance with embodiments of theinvention to implement selection logic, in addition to combinational andselection logic. As used herein, a “selection operation” refers to afunction in which one or more of a plurality of inputs are passed asoutputs. In certain embodiments, the selection operation passes one of aplurality of inputs as an output. Such a selection operation differsfrom a sequential-logic operation because it does not depend on a pasthistory of the inputs. It also differs from combinational-logicoperations, which do not require that the output correspond to one ofthe inputs. This is easily seen for an NAND gate, which produces anoutput 1 in response to two 0 inputs; the output does not correspond toeither of the inputs. It is also true, however, for an OR gate. Althoughin every instance the output of an OR gate is equal to one of theinputs, the gate does not act to pass one of the inputs as an output;instead, a combinational mapping is performed from the inputs to theoutputs that happens to include some commonality. In addition, selectionoperations are not limited to instances in which the number of inputs istwo and/or the number of outputs is one. More generally, any pluralityp_(in) (≧2) of inputs may be accepted, of which a number p_(out) (>1)are passed.

[0107] Embodiments of the invention also do not limit the inputs and/oroutputs to the base Boolean values 0 and 1. As noted in connection withTables X and XI and the associated syntactic expressions, cells used inembodiments of the invention may implement operations in whichtruth-table entries are instead functions of such base Boolean values.In this respect, the invention includes embodiments that provide for theimplementation of Boolean functionals, which are defined herein asoperations that admit functions of Boolean variables among their inputsand/or outputs, in addition to admitting base Boolean values among theirinputs and/or outputs.

[0108] Both the formalism presented herein and the implementation withthe cells described above permit a further generalization that increasesthe flexibility of digital design and its optimization. Such ageneralization may be understood with reference to what are definedherein as higher-order Boolean functions. Conventional digital circuitdesign uses only what are referred to herein as zero-order Booleanfunctions, which admit only base Boolean values among their input(s)and/or output. In contrast, some embodiments of the invention use afirst-order Boolean function, which corresponds to a Boolean functionaland admits zero-order Boolean functions in addition to base Booleanvalues among its inputs and/or output(s). Other embodiments use asecond-order Boolean function, which admits first-order Booleanfunctions, zero-order Boolean functions, and base Boolean values amongits inputs ard/or output(s). In still other embodiments of theinvention, even greater orders of Boolean functions are used, suchorders admitting all lower orders of Boolean functions among theirinputs and/or output(s) in addition to admitting the base Boolean valuesused in conventional design. All orders of Boolean functions other thanzero-order Boolean functions are sometimes referred to hereincollectively as “higher-order” Boolean functions.

[0109] This generalization may be illustrated with an example based onthe C=A+{overscore (B)} example discussed earlier:

[0110] J .UB G H VDD VSS F C

[0111] F .UB D E VSS C VDD C

[0112] C .UB A B VDD VDD VSS VDD

[0113] In this example, the third expression corresponds to thezero-order function C=A+{overscore (B)}, which admits only base Booleanvales 0 and 1 among its arguments. Such a function uses a combinationalmapping and may be implemented using gates, such as with an OR gate anda NOT gate. The second expression corresponds to a first-order function,or functional, that admits the zero-order function C as one of itsarguments, in addition to admitting the base Boolean values. The firstexpression corresponds to a second-order function that admits thefirst-order function F, the zero-order function C, and the base Booleanvalues among its arguments. The first and second expressions thus eachcorrespond to expressions for higher-order functions. All three of theexpressions may be implemented in embodiments of the invention using thecells as described above.

[0114] One effect of the ability to use higher-order functions may beseen with a comparison to the exclusive use of Boolean operations. SuchBoolean operators operate only on the base Boolean values 0 or 1, or onvariables that represent the base Boolean values 0 or 1, i.e. that havealready had a value of 0 or 1 bound to it. Boolean minimization oroptimization techniques are based on decomposing the expressions beingminimized to consider the meaningful possible combinations of assignmentof 0 or 1 to each Boolean variable (with the possible existence of“don't care” states for some variables under some circumstances reducingthe meaningful possible combinations downward from the set of allpossible combinations). Higher-order functions allow one to optimize, orminimize a circuit, without the requirement to decompose the functionresult to each possible value and considering each separately. In otherwords, when a circuit with the algorithms described below, one need notknow what the value of the functions or variables are; optimization isperformed regardless. In conventional methods limited to the use ofBoolean operators, each variable and function is decomposed into allpossible values for the functions and variables, i.e. to define acomplete truth table, before any optimization can be performed; in suchconventional methods one must exhaustively assign a value to allvariables and functions.

[0115] IV. Optimization

[0116] The expanded availability of logical operations provided byembodiments that use cells based on the ULGs and as represented by theformalism described permits increased optimization. In many instances,these logical operations may be used to determine optimized methods ofimplementing a given function. A number of such logical operations areillustrated, and it will be understood by those of skill in the art thatstill other logical operations may derive from the formalism in otherembodiments of the invention. Moreover, while the formalism is used as amatter of convenience to illustrate the nature of the optimizations, itwill be understood that all the expressions that follow may beimplemented using the previously described cells in the mannerexplained. This is true even in instances where the expressionscorrespond to functions not accessible by standard Boolean logic. Insome cases, use of the formalism shows how multiple manipulations may beperformed to achieve an optimization, it being necessary only toimplement the final result with the previously described cells toachieve the optimized function.

[0117] A number of the operations that may be performed with logicalfunctions as expressed using the formalism described herein aresummarized in FIGS. 9A-9D, which provide flow diagrams to explain howsome such operations may be performed. In different embodiments, variouscombinations of one or more such operations may be performed and theinvention is not limited to any particular order or number of suchoperations. Accordingly, each of FIGS. 9A-9D indicates that it may beentered as part of a greater flow of operations from a previoustransformation. It is not necessary that a previous transformationnecessarily have been performed in any case, although the indication isincluded to note that some embodiments of the invention contemplate thatprevious transformations may have been performed. Also, while the flowdiagrams in each of FIGS. 9A-9D shows an exemplary order in whichoperations may be performed, such an ordering is not necessary andalternative embodiments permit alternative orderings. Moreover, in someembodiments, it is possible that some operations may be performedsimultaneously, such as when different parts of a large structure areoptimized at the same time.

[0118]FIG. 9A summarizes a number of operations that may collectively beconsidered to correspond to the removal of inversions. Accordingly, themethod shown in FIG. 9A begins at block 902 with a syntactic expressionfor a cell, perhaps, but not necessarily, after certain previous logicaltransformations have been effected. At block 904, an identification ismade whether there are any inversions in the syntactic expression forremoval. If not, the method proceeds to a potentially subsequenttransformation at block 906. In the event that it is desirable to removean inversion, the method may proceed along one of at least threebranches depending on the type of inversion. Branch 903 corresponds toinversions in the control or state variables Y; branch 905 correspondsto inversions in the truth table; and branch 907 corresponds toinversions in the buffer.

[0119] In one embodiment, inversions of the control or state variablemay proceed at block 908 by interchanging adjacent groups in the truthtable. The size of the groups to be interchanged depends on thesignificance of the control or state variable to be inverted. Thus, if acontrol Y[k] is to be inverted, groups of size 2^(k) are inverted. Thismay be illustrated by considering a cell comprising a ULG and a buffer:

[0120] Q .UB Y[2] Y[1] YN[0] A B C D E F G H;

[0121] In this instance, the least significant control Y[0], defined byk=0, is to be inverted so that adjacent states are interchanged:

[0122] Q .UB Y[2] Y[1] Y[0] B A D C F E H G;

[0123] In a similar fashion, when k=1 for the control to be inverted,adjacent pairs of states are to be interchanged:

[0124] Q .UB Y[2] YN[1] Y[0] A B C D E F G H;

[0125] Q .UB Y[2] Y[1] Y[0] C D A B G H E F;

[0126] When k=2 for the control to be inverted, adjacent quads of statesare to be interchanged:

[0127] Q .UB YN[2] Y[1] Y[0] A B C D E F G H;

[0128] Q .UB Y[2] Y[1] Y[0] E F G H A B C D;

[0129] The flow diagram in FIG. 9A provides a loop back to block 904after a particular control has been inverted by interchanging states.This contemplates the possibility of performing inversions on multiplecontrols, which are therefore effected by performing the relevantinterchanges in succession. The interchanges are commutative so that theresulting syntactic expression is independent of the order in which theyare performed:

[0130] Q .UB YN[2] YN[1] YN[0] A B C D E F G H;

[0131] Q .UB Y[2] Y[1] Y[0] H G F E D C B A;

[0132] The resulting expression, which may be implemented using thecells as described above, follows from any order of performing thecontrol inversions and respective truth-table interchanges.

[0133] The flow diagram of FIG. 9A notes at block 910 that in someinstances the removal of inversions may be accomplished by entering thecontrol as a map-entered variable instead of performing interchanges. Insuch cases, the ability of embodiments of the invention to accommodatenon-Boolean selection operations is exploited to achieve greater levelsof optimization. Within the syntax used to illustrate the principlesdescribed herein, the identification of an inverted control YN with asequence VSS VDD permits removal of the inversion by entering thecontrol into the truth table:

[0134] Q .UB Y[1] YN[0] VSS VDD A A;

[0135] Q .UB Y[1] Y[0] A;

[0136] As indicated, entry of the control in the truth table willusually also require a repetition of a state A at the appropriate level.

[0137] In some cases, it may be desirable to permute the truth table toachieve such VSS VDD sequences by permuting the control:

[0138] Q .UB YN[1] Y[0] VSS A VDD A;

[0139] Q .UB Y[0] YN[1] VSS VDD A A;

[0140] Q .UB Y[0] Y[1] A;

[0141] In the above progression, optimization of the cell is achieved bynoting that the sequence VSS VDD may be achieved through a permutationof the truth table and by noting the repetition of state A. Permutingthe controls results in a truth table identical to that of the precedingexample, and therefore the least significant control may become amap-entered variable.

[0142] The same principles apply with more significant levels ofcontrol, for which optimization may remove an inversion by entering themore significant control as a map-entered variable under somecircumstances. In one embodiment, for example, this is achieved when theless significant controls are redundant:

[0143] Q .UB Y[2] YN[1] Y[0] A A A A VSS VSS VDD VDD;

[0144] Q .UB Y[2] A Y[1];

[0145] The optimization has been achieved by recognizing the existenceof an analogous pattern, namely the repetition of A at a highersignificance level and the existence of the sequence VSS VSS VDD VDD. Itwill now be evident to those of skill in the art that permutation ofcontrol may be used to restructure the truth table to identify suchsequences and thereby optimize the function by removing the inversion.In addition, it will also be evident that these principles may beapplied to any significance level for the control. For example, aneightfold repetition of A coupled with the sequence VSS VSS VSS VSS VDDVDD VDD VDD will permit removal of a YN[2] inversion, perhaps afterpermuting the controls to achieve such a sequence in the truth table.

[0146] At block 912 of FIG. 9A, it is noted that inversion of thetruth-table states may be achieved by inverting the buffer in the cell:

[0147] Q .UBN Y[0] AN BN;

[0148] Q .UB Y[0] A B;

[0149] By inverting the buffer, all entries in the truth table areinverted. Block 914 notes the converse function in which the buffer isinverted by inverting all elements of the truth table. While thefunctional effect of blocks 912 and 914 is identical, they areconceptually converse because in one instance the goal of inverting thetruth table is achieved by inverting the buffer and in the otherinstance the goal of inverting the buffer is achieved by inverting thetruth table.

[0150] Every operation in the FIG. 9A cycles back to block 904. As notedwith respect to block 908, this aspect emphasizes that multiple of thesetransformations may be used in effecting optimizations and that they maybe performed in different orders. For example, for some cells,optimization might be achieved by: (1) first, inverting a buffer toinvert a truth table; (2) second, permuting the control so that theresulting truth table includes sequences that permit the entry ofcontrol variables; and (3) finally, performing interchanges within thetruth table to remove other control inversions.

[0151] The entry of a control variable into a truth table as amap-entered variable, such as discussed with respect to block 910 inFIG. 9A not only has the effect of removing an inversion, but alsoreduces the number of nodes in the cell. There are other truth-tablesequences that permit optimization by accepting the entry of controlvariables and thereby reducing the number of nodes. The flow diagram inFIG. 9B provides a general explanation of how such sequences may beachieved. Essentially, the same procedures are followed as discussedwith respect to block 910 for inversions: a truth-table having elementsof certain sequences is identified and permuted to realize thosesequences, which are then optimized by entering the correspondingcontrol variable. A simple example corresponds to the example discussedwith respect to block 910, but without the inversion:

[0152] Q .UB Y[1] Y[0] VDD VSS A A;

[0153] Q .UB Y[1] Y[0] A;

[0154] In this example, a repetition of A with the sequence VDD VSS(instead of the sequence VSS VDD) is sufficient to enter theleast-significant control variable into the truth table. This is donewith the control variable directly, instead of with its inversion as wasdone in block 910.

[0155] Thus, the general procedure illustrated in FIG. 9B begins atblock 916 with a syntactic expression for a cell, with the figure notingthat it is possible (but not required) in some embodiments for certainother transformations to have been performed previously with thesyntactic expression. At block 918, existing redundancies in the controlare removed. Such redundancies are manifested by a repetition in thetruth table at the level of the redundant control, i.e. in groups ofsize 2 ^(k) for control Y[k]. A trivial case occurs for the lowest levelof control:

[0156] Q .UB Y[0] A A;

[0157] Q .B A;

[0158] This example is trivial because the cell does nothing other thanproduce the result A for every input; control is unnecessary and may beremoved entirely. The same principle applies, however, for higher levelsof control. For example, when k=1, the repetition of pairs of states maypermit the removal of Y[1]:

[0159] Q .UB Y[1] Y[0] A B A B;

[0160] Q .UB Y[0] A B;

[0161] This result simply uses the fact that the higher control levelhas no effect, with the output of the cell depending solely on the leastsignificant control. Similarly, when k=2, the repetition of quads ofstates may permit the removal of Y[2]:

[0162] Q .UB Y[2] Y[1] Y[0] A B C D A B C D;

[0163] Q .UB Y[1] Y[0] A B C D;

[0164] This result expresses the fact that the output of the cell isdependent only on the two lowest control levels and that the highestcontrol has no effect. These principles may be extended to still largerrepeated blocks and the consequent removal of still more significantcontrol levels.

[0165] At block 920, permutations may be performed in the control torearrange the truth table to identify sequences that permit the entry ofthe control variables. For the entry of a lowest level control,sequences of VDD VSS, coupled with a pair of repeated variables A A, issought. For the entry of the next level control, sequences of VDD VDDVSS VSS, coupled with four repeated variables A A A A, is sought. Forthe next level control, sequences of VDD VDD VDD VDD VSS VSS VSS VSS,coupled with eight repeated variables A A A A A A A A, is sought.Similar sequences for still higher control levels follow the samepattern. At block 922, the control variable(s) are entered into thetruth table to account for these patterns.

[0166] Thus, one example of applying blocks 920 and 922 is as follows:

[0167] Q .UB Y[2] Y[1] Y[0] VDD VDD VSS VSS A B A B;

[0168] Q .UB Y[2] Y[0] Y[1] VDD VSS VDD VSS A A B B;

[0169] Q .UB Y[2] Y[0] Y[1] Y[1] A B;

[0170] In the initial syntactic expression, the sequence VDD VDD VSS VSSappears, but it is not possible to remove the k=1 control because thereis no corresponding sequence of four repeated variables. The existenceof duplicates of both A and B, however, suggests that the controls maybe permuted to achieve sequences of VDD VSS coupled with pairs ofrepeated variables. This is achieved in the second line by permutingY[1] and Y[0]. Accordingly, it is possible in the third line to entertwo occurrences of the Y[1], which is now the lowest level of control,into the truth table. The corresponding cell is therefore optimized byreducing the number of nodes and entering the original Y[1] controlvariable into the truth table.

[0171] Block 924 notes that the process of identifying and removingredundancies and permuting control variables to permit their entry intothe truth table may be repeated to achieve further optimizations. Themethod is thus looped until these procedures have optimized thesyntactic expression in this way as much as desired. At block 926, themethod thus proceeds to another type of transformation, if desired, toeffect further optimizations.

[0172] In addition to reducing nodes for a single cell, it is possiblein embodiments of the invention to provide optimizations by combiningnodes from multiple cells. The flow diagram shown in FIG. 9C provides amethod corresponding to one embodiment for combining nodes. The methodbegins at block 928 with syntactic expressions for multiple cells. Asfor the other optimization procedures, FIG. 9C notes explicitly thatprior transformations may have taken place on these syntacticexpressions, although this is not required. In considering whether nodescan be combined, a check is made at block 930 whether any of thesyntactic expressions outputs a control variable present in another ofthe expressions. If so, the control variables are converted intomap-entered variables at block 932. Examples of combining nodes in whichsuch conversions are used are provided below, but the principles ofcombining nodes are initially illustrated for cases where there is nosuch conversion.

[0173] One method for combining nodes is thus summarized by blocks 934and 936 in which the control for the multiple syntactic expressions iscombined and then states in the merged expression are defined. Forexample, consider the following two syntactic CHDL expressions in whichthe output of the second expression, A, is one of the inputs to thefirst expression:

[0174] Q .UB YQ A B;

[0175] A .UB YA C D;

[0176] The expressions are merged, and the nodes thereby combined, inthe following way. First, the control is combined at block 934 by addingthe control for the second expression to the first expression—YQ thenfunctions as a k=1 level control and YA functions as a k=0 levelcontrol:

[0177] Q .UB YQ YA A A B B;

[0178] As can be seen, increasing the level of the YQ control by asingle level to k=1 acts to duplicate each of the truth-table entries.The order in which the controls were combined was determined by therelationship between the inputs and outputs of the expressions.Specifically, since the expression for Q has the output of theexpression for A as an input, YQ was made the higher-level control andYA the lower-level control. After combining the controls, the states aredefined in the merged expression in accordance with the expressions atblock 936:

[0179] Q .UB YQ YA C D B B;

[0180] In this instance, the sequence A A is substituted with C D inaccordance with the syntactic expression for A. The final expressionachieves the optimization by permitting implementation of the resultingexpression with a single cell as described above.

[0181] While this example showed how two expressions could be merged, itmay be applied more generally to any number of expressions. For example,the following three expressions may be merged in a similar fashion witha sequential process:

[0182] Q .UB YQ A B;

[0183] A .UB YA C D;

[0184] B .UB YB E F;

[0185] The second and third expressions both have outputs thatcorrespond to inputs of the first expression. Accordingly, in combiningcontrol pursuant to block 934, the control of the first expression isused as the highest level control. First, the first and secondexpressions are merged by combining their control and defining thestates in the merged expression in the same way as for thetwo-expression example:

[0186] Q .UB YQ YA A A B B;

[0187] Q .UB YQ YA C D B B;

[0188] Subsequently, the third expression is merged into thiscombination. First, the additional control causes YQ to become a k=2level control and YA to become a k=1 level control, with YB remaining asa k=0 level control:

[0189] Q .UB YQ YA YB C C D D B B B B;

[0190] As seen in this expression, the additional level of controlcauses a duplication of each of the truth-table elements. The states inthis expression are now defined according to block 936 in terms of theoriginal third expression by substituting pairs of B's with the sequenceE F:

[0191] Q .UB YQ YA YB C C D D E F E F;

[0192] This result thus corresponds to an expression that combines theoriginal three expressions and may be implemented as a cell in themanner described above.

[0193] Both of these examples have begun with expressions thatcorrespond to ULG cells that may be implemented with multiplexors of thesame size. There is, however, no limitation on embodiments of theinvention that requires that they be the same size. It is possible toperform optimizations for combining nodes that correspond to merging asmaller multiplexor into a larger multiplexor or to merging a largermultiplexor into a smaller multiplexor. This may be seen in thefollowing examples in which each of the initial expressions correspondsto a different-sized multiplexor when such an implementation is used.For example, in the set

[0194] Q .UB YQ[1] YQ[0] A B C D;

[0195] A .UB YA E F;

[0196] the second expression has an output that is used as an input inthe first expression, and corresponds to a smaller-sized multiplexorthan does the first expression. The nodes are combined in the samefashion already described. First, control is combined in accordance withblock 934:

[0197] Q .UB YQ[1] YQ[0] YA A A B B C C D D;

[0198] Subsequently, states are defined in the merged expression, inthis instance by substituting pairs of A's with E F:

[0199] Q .UB YQ[1] YQ[0] YA E F B B C C D D;

[0200] This final expression may thus be implemented as a cell andachieves optimizations resulting from merging the smaller multiplexorinto the larger multiplexor.

[0201] It is similarly possible to combine nodes in a fashion thatcorresponds to merging a larger multiplexor into a smaller multiplexorin embodiments that use multiplexors:

[0202] Q .UB YQ A B;

[0203] A .UB YA[1] YA[0] C D E F;

[0204] In this example the output of the expression corresponding to thelarger multiplexor is an input to the expression corresponding to thesmaller multiplexor. The nodes are combined in the same way, by firstcombining the control of the two expressions in accordance with block934. Since two levels of control from the second expression are to becombined with the first expression, YQ becomes a k=2 level control:

[0205] Q .UB YQ YA[1] YA[0] A A A A B B B B;

[0206] Subsequently, the states are defined in accordance with block 936by substituting quads of A's with C D E F as dictated by the secondoriginal expression:

[0207] Q .UB YQ YA[1] YA[0] C D E F B B B B;

[0208] Each of these examples illustrates how to combine nodes indifferent circumstances where the output of one of the expressions isone of the inputs to another of the expressions. In some cases, however,the output of one of the expressions may be one of the controls ofanother expression, a condition checked for a block 930. In suchinstances, the control variable is converted into a map-entered variableat block 932 before combining control and defining states. This may beillustrated with the following two examples, the first of whichcorresponds to an AND sub-function and the second of which correspondsto an OR sub-function.

[0209] Thus, consider merging nodes for the following two syntacticexpressions:

[0210] Q .UB YQ[1] YQ[0] A VDD VSS VSS;

[0211] YQ[1] .UB YA B C;

[0212] The circumstance in this example differs from the previousexamples because the output of the second expression, YQ[1], is acontrol of the first expression and not an input in the truth tableentries. The ability of embodiments to accept variables in thetruth-table elements is thus exploited to re-express the firstexpression with YQ[1] in the truth table. First, the control variablesare permuted so that YQ[1] is the least significant control:

[0213] Q .UB YQ[0] YQ[1] A VSS VDD VSS;

[0214] Then, it is recognized that with YQ[1] as the least significantcontrol, the VDD VSS sequence in the I₁-I₀ positions simply correspondsto YQ[1]. It is also recognized that in the I₃ position, YQ[1] and A areequivalent. Accordingly, after converting control variables tomap-entered variables pursuant to block 932, the expression may bewritten

[0215] Q .UB YQ[0] A YQ[1] VSS YQ[1] YQ[1];

[0216] Essentially, this conversion recognizes the equivalence of truthtables XIIA and XIIB:

[0217] Combining control with the second expression at block 934 resultsin

[0218] Q .UB YQ[0] A YA YQ[1] YQ[1] VSS VSS YQ[1] YQ[1] YQ[1] YQ[1];

[0219] Finally, defining states at block 936 so that pairs of YQ[1] aresubstituted with B C as required by the original second expressionresults in the merged expression

[0220] Q .UB YQ[0] A YA B C VSS VSS B C B C;

[0221] The same procedure may also be used for the following twosyntactic expressions:

[0222] Q .UB YQ[1] YQ[0] VDD VDD A VSS;

[0223] YQ[1] .UB YA B C;

[0224] In this example, the expression for YQ[1] is the same as in theprevious example, but the expression for Q is different. Permuting thecontrol variables so that YQ[1] is least significant,

[0225] Q .UB YQ[0] YQ[1] VDD A VDD VSS

[0226] Recognizing that the VDD VSS sequence in the I₁-I₀ positionscorresponds to YQ[1] and that YQ[1] and A in the I₂ position areequivalent results in

[0227] Q .UB YQ[0] A VDD YQ[1] YQ[1] YQ[1];

[0228] This conversion effectively recognizes the equivalence of truthtables XIIIA and XIIIB:

[0229] Combining control with the second expression according to block934 results in

[0230] Q .UB YQ[0] A YA VDD VDD YQ[1] YQ[1] YQ[1] YQ[1] YQ[1] YQ[1];

[0231] Finally, defining states at block 938 so that pairs of YQ[1] aresubstituted with B C as required by the original second expressionresults in the merged expression

[0232] Q .UD YQ[0] A YA VDD VDD B C B C B C;

[0233] It is noted that in certain instances, the method outlined inFIG. 9C may be combined with moving flip flops forward to facilitatereductions. For example, consider application of the method to thefollowing set of expressions

[0234] Q .UB Y[2:1] A B C D;

[0235] A .UD Y[0] E F CLK;

[0236] B .UD Y[0] G H CLK;

[0237] C .UD Y[0] I J CLK;

[0238] D .UD Y[0] K L CLK;

[0239] As previously mentioned, the notation Y[2:1] is equivalent to theexpression Y[2] Y[1]. This set of expressions could be implemented usingfive cells, one that comprises a ULG and a buffer and four that comprisea ULG and a F/F. The result of moving the flip flops forward is

[0240] Q .UD Y[2:1] A B C D CLK;

[0241] A .UB Y[0] E F;

[0242] B .UB Y[0] G H;

[0243] C .UB Y[0] I J;

[0244] D .UB Y[0] K L;

[0245] Accordingly, applying the method of FIG. 9C to combine the nodesresults in

[0246] Q .UD Y[2:0] E F G H I J K L CLK;

[0247] This expression may be implemented with a cell comprising a ULGand a D F/F in certain embodiments of the invention.

[0248] Thus, the method outlined in FIG. 9C permits nodes to be reducedfrom multiple syntactic expressions. This includes a variety ofdifferent circumstances, including cases where there are arbitrarilymany syntactic expressions, where some of the expressions correspond toimplementations of different sizes, and cases where some of theexpressions have outputs that correspond to either inputs or controls ofother expressions. After nodes have been merged, the method may proceedto another transformation at block 938, although this is not arequirement.

[0249] Additional optimization functions may be realized by using theset and reset facilities that are provided in the formalism and whichmay be implemented by using those facilities in cells according toembodiments of the invention. The use of set and reset facilities aresummarized in FIG. 9D and arise primarily when half the truth tablestates are either high or low. Thus, a method for optimizing begins witha syntactic expression for a cell at block 940, with FIG. 9D notingexplicitly that previous transformations may also have been performed onthe cell expression, although this is not a requirement. A check is madeat block 944 whether half the truth table states are low. If so, thecontrol variable may be changed to a reset to a F/F in accordance withblock 946. For example, consider the syntactic function

[0250] Q .UD Y[2:0] VSS VSS VSS VSS A B C D CLK;

[0251] In this example, a cell that implements this function comprises aULG and a D F/F. As previously mentioned, the notation Y[2:0] isequivalent to Y[2] Y[1] Y[0]. Half of the truth table states in thisexpression are low, i.e. VSS, so that the highest level control may beentered into the map as a reset input to the F/F:

[0252] Q .UDR Y[1:0] A B C D Y[2] CLK (RST=‘C’);

[0253] The name of the cell explicitly notes that a resetable D F/F (DR)is used and the reset has been noted. This function may then beimplemented using the cells described above.

[0254]FIG. 9D notes at block 942 that in some instances it may be usefulto perform permutations of the control variables to put the truth tableinto a form that allows using the reset input for optimization. If halfthe truth table states are low, but do not appear as a group, they maybe aligned with the permutations. For example, the function

[0255] Q .UD Y[1] Y[0] VSS A VSS B CLK;

[0256] has half of its truth table states low, but they are not aligned.Interchanging the control variables aligns them

[0257] Q .UD Y[0] Y[1] VSS VSS A B CLK;

[0258] so that the most significant control variable may be entered intothe map at block 946 as a reset input to the F/F:

[0259] Q .UDR Y[1] A B Y[0] CLK (RST=‘C’);

[0260] Note that in this example, the lack of alignment among the lowstates has resulted in Y[0] being entered into the map as part of theoptimization rather than Y[1].

[0261] Similar optimizations may be achieved if half the truth tablestates are high, as checked at block 948. If so, permutations of thecontrol variables may be performed at block 950 to align the high statesand the control variable entered into the map as a set input to a F/F atblock 952. For example, consider the function

[0262] Q .UD Y[1:0] VDD A VDD B CLK;

[0263] Half of the states in the truth table are high, i.e. VDD, so thatoptimization with a set input may be achieved. Permuting the controlvariables to align the high states in accordance with block 950 resultsin

[0264] Q .UD Y[0] Y[1] VDD VDD A B CLK;

[0265] Entering the most significant control as a map-entered variableas a set input to the F/F results in

[0266] Q .UDS Y[1] A B Y[0] CLK (SET=‘C’);

[0267] This function may then be implemented using the cells describedabove in an embodiment.

[0268] Still other combinations of expansion, inversion, and/orpermutation may be used to achieve the conditions for using set or resetfor optimization. An example that illustrates several of theoptimization manipulations discussed above begins with the syntacticexpression

[0269] Q .UD AN VSS NOT_RESET CLK;

[0270] Optimization of this function may proceed by first expanding theNOT_RESET as a control variable:

[0271] Q .UD AN NOT_RESET VSS VSS VDD VSS CLK;

[0272] Inversion of the two control variables in accordance with FIG. 9Aresults in two interchanges of the truth table, a first interchangebased on single entries and a second interchange based on pairs:

[0273] Q .UD A RESET VSS VDD VSS VSS CLK;

[0274] Interchanging the controls to prepare for entering A as amap-entered variable results in

[0275] Q .UD RESET A VSS VSS VDD VSS CLK;

[0276] The presence of the VDD VSS sequence and the repeated VSS elementpermits the A control variable to be entered into the map in accordancewith FIG. 9B:

[0277] Q .UD RESET VSS A CLK;

[0278] It is now apparent that since there are only two states in thetruth table and one of them is low, that half the states are low and thecontrol variable may be entered into the map as a reset input to theF/F:

[0279] Q .DR A RESET CLK (RST=‘C’);

[0280] Since all of the control variables have been entered into themap, there is no need for a ULG element in implementing this optimizedexpression. Instead, it may be implemented in one embodiment using onlya resetable D F/F, even though the original expression corresponded toan implementation comprising both a ULG and a D F/F.

[0281] Each syntactical expression of the formalism may be viewed as anentry for a ULG netlist that defines a digital circuit. The ULG netlistuses basic cells and follows the CHDL syntax. Such a ULG netlist may beat least partially optimized by successively performing some of theindividual manipulations described above. In some embodiments, theresulting optimized ULG netlist may be implemented directly using thecells described above. In other embodiments, however, the manipulationof the ULG netlist may be viewed as intermediate step in optimizing adigital design that has been expressed in another netlist format. Insuch instances, the other netlist format using another cell libraryand/or another syntax (e.g., VHDL or Verilog) is initially translated tothe ULG netlist format. After performing some optimization steps, theoptimized ULG netlist may be translated back into the original formatfor implementation. In this way, an embodiment is provided that achievesoptimization of digital designs within preexisting netlist formats.

[0282] There are a variety of ways in which the individual manipulationsof the syntactic expressions comprised by a ULG netlist may be performedand the degree to which the corresponding digital design is optimizedmay depend on how those manipulations are executed. While in someinstances it is possible for the manipulations to be performed by handby a digital designer, it is expected that at least some level ofautomation may be used. In one embodiment, for example, a computerprogram may be provided that allows a digital designer to select thetypes of manipulations to be performed and sections of the ULG netliston which to perform them, with the execution of the manipulations beingperformed automatically. In another embodiment, a predeterminedalgorithm is used in a completely automated way to perform themanipulations or optimizations.

[0283] One example of such a predetermined algorithm is illustrated withthe flow diagram provided in FIG. 10, although it will be appreciated bythose of skill in the art that numerous other algorithms mayalternatively be used. In some specific instances, alternativealgorithms use the same manipulations but perform them in a differentorder. In some other specific instances, alternative algorithms use adifferent set of manipulations. The algorithm shown in FIG. 10 may beginat block 1004 by translating an existing netlist to a ULG netlist if thedesign to be optimized was not initially created using the ULGformalism. In one embodiment, translating from the existing netlist tothe ULG netlist is performed as a one-to-one translation betweensyntactic expressions. For one embodiment, this translation may beviewed conceptually in terms of the elements used in that embodiment toimplement the original and ULG netlists—every logic gate, such as NAND,OR, etc. is converted into a multiplexor-based implementation amenableto optimizations provided by the syntactic manipulations discussedabove.

[0284] At block 1008, data and control elements are discerned in theresulting ULG netlist. The distinction between such data and selectioncontrol elements was previously discussed with respect to Table VIII. Inone embodiment, such discernment may be performed with a high-leveldesign language. In certain embodiments, there are additional advantagesin the subsequent optimization where the data and control are identifiedfrom the original behavioral netlist. Once the data and control elementshave been identified, the operations identified in blocks 1016-1068 maybe performed by using the syntactic manipulations described above. Thus,at block 1016, connection cells to base Boolean values are removed byincorporating the corresponding functions into the syntacticexpressions. Similarly, non-inverting buffers are removed at block 1020and also incorporated directly into the syntactic expressions. At block1024, inverters are removed by syntactically inverting the relevant dataelements in individual syntactic expressions. The removal of redundantnodes at block 1028 may proceed by reducing, combining, and permutingnodes in the syntactic expressions. Methods for such reductions,combinations, and permutations for certain embodiments were describedabove in connection with FIGS. 9B and 9C. At block 1032, inversions areremoved. The inversions removed may include control/state variableinversions, truth table inversions and buffer inversions, for whichmethods of removing were described above in connection with FIG. 9A.

[0285] A loop comprising blocks 1036 and 1040 may be executed toidentify and merge fanout nodes. A “fanout node” describes aconfiguration in which a single output is directed to multiple parts oftruth tables and, in some instances, optimizations may be realized bymerging such fanout nodes. A check is first performed at block 1036 toidentify whether the ULG netlist includes any fanout nodes, in whichcase they are reduced at block 1040 by performing permutations andreductions as described in connection with FIG. 9C. The loop betweenblocks 1036 and 1040 is included because the reductions performed atblock 1040 may produce some new fanout nodes that may be identified andmerged through additional permutations and reductions. The method thusloops until no fanout nodes are identified at block 1036.

[0286] After the fanout nodes have thus been merged, a check is made atblock 1044 to identify syntactic expressions corresponding tosynchronous cells in which half the truth-table states are low. If suchan expression is identified, the corresponding control variable is movedto be a reset input at block 1048. A method for doing so is described inconnection with FIG. 9D and may include performing permutations to alignthe low truth-table states. A similar check is made at block 1052 toidentify syntactic expressing corresponding to synchronous cells inwhich half the truth-table states are high. If such an expression isidentified, the corresponding control variable is move to be a set inputat block 1056. A method for doing so is also described in connectionwith FIG. 9D and may include performing permutations to align the hightruth-table states.

[0287] At block 1060, the syntactic expressions are grouped by commoninputs. Such grouping permits identification of subfunctions at block1064. The common subfunctions have shared characteristics that may beextracted before reducing other nodes. At block 1068, buffers are madeto be inverting buffers.

[0288] If the original ULG netlist produced at block 1004 wasnonoptimal, performing the above manipulations may produce a differentULG netlist that is amenable to implementation with smaller area,greater speed, and/or lower power requirements. In some embodiments, itis possible that not all of the manipulations will be performed, thatsome may be performed multiple times, and that they may be performed ina different order than described. Once the method has produced a new ULGnetlist, it may be implemented at block 1072 by translating thesyntactic expressions in the ULG netlist with the cells described above.In such embodiments, the method functions not only to optimize thedigital design but also to provide a multiplexor-based implementation ofit. In some alternative embodiments, the resulting ULG netlist may beamenable to translation back into the original netlist syntax forimplementation using Boolean logic gates. In these embodiments, themethod may be viewed as providing an optimization of a digital designwhile retaining its underlying or structural characteristics.

[0289] In some embodiments, it is desirable for the techniques used forthe syntactic manipulations to be embedded in an optimization tool orsynthesizer. Accordingly, the methods of the invention for converting anetlist into a ULG netlist and for optimizing the ULG netlist may beperformed by a computer, one example of a suitable configuration forwhich is shown in FIG. 11. This figure broadly illustrates howindividual system elements may be implemented in a separated or moreintegrated manner. The computer 1100 is shown comprised of hardwareelements that are electrically coupled via bus 1112, including aprocessor 1102, an input device 1104, an output device 1106, a storagedevice 1108, a computer-readable storage media reader 1110 a, acommunications system 1114, a processing acceleration unit 1116 such asa DSP or special-purpose processor, and a memory 1118. Thecomputer-readable storage media reader 1110 a is further connected to acomputer-readable storage medium 1110 b, the combination comprehensivelyrepresenting remote, local, fixed, and/or removable storage devices plusstorage media for temporarily and/or more permanently containingcomputer-readable information. A communications system 1114 may comprisea wired, wireless, modem, and/or other type of interfacing connection.

[0290] The computer 1100 also comprises software elements, shown asbeing currently located within working memory 1120, including anoperating system 1124 and other code 1122, such as a program designed toimplement optimization methods of the invention. It will be apparent tothose skilled in the art that substantial variations may be used inaccordance with specific requirements. For example, customized hardwaremight also be used and/or particular elements might be implemented inhardware, software (including portable software, such as applets), orboth. Further, connection to other computing devices such as networkinput/output devices may be employed.

[0291] Example: The method described with respect to FIG. 10 was used tooptimize a netlist for a microcontroller. A synthesized netlist ofcommercial library cells was translated to the ULG netlist formalism andoptimized by performing the described syntactic manipulations. The sizeof implementing the optimized ULG netlist with the cells describedherein was then compared with the size of the original implementation.The overall size was reduced by about 37%, a significant reduction.

[0292] V. Digital Design with Syntactic Manipulation

[0293] Referring next to FIG. 12, a flow diagram of an embodiment of aprocess for preparing a ULG ASIC cell library is shown. In thisembodiment, a new fabrication process is being adapted to use the kernelcell/basic cell topology. The depicted portion of the process begins instep 1204 where a layout engineer or technician analyzes the layoutrules for the target fabrication process. The fabrication process couldhave different amounts of metalization layers, different semiconductorcompositions, different transistor types, and different topologies suchas SOI, etc. that are considered during layout. The kernel cells arelaid out, at least partially, by hand to optimize the circuitry in step1208. Some embodiments could start the kernel cell layout with acomputer routed design that is hand-customized. Care is taken to havekernel cells of a consistent height and to adhere to abutment guidelinessuch that clock signals and some of the I/O are latitudinally aligned.Other embodiments could rely upon autorouting entirely to build thekernel cells.

[0294] The kernel cells can be assembled in a number of ways topotentially create hundreds of basic cells 100 in the ULG ASIC celllibrary. The basic cell configurations are specified in step 1212. Thiscould be done by editing a script fed to the tool that combines thekernel cells into basic cells. In step 1216, the kernel cells areautomatically assembled into basic cells 100 in this embodiment. Someembodiments could assemble the kernel cells manually or in-art manually.Once the basic cells are generated, they are verified in step 1220. Thisverification validates the digital and analog performance of the basiccells. Any problems uncovered in verification can be fixed in the kerneland/or basic cells. To complete the ULG ASIC cell library, anyspecialized cells are laid-out in step 1224.

[0295] In one embodiment, selection logic is used in digital circuits toreplace some or all combinatorial logic. A significant proportion of thebasic cells use a ULG 104. In contrast, conventional semiconductorcircuits do not use ASIC cell libraries that include ULG circuits. Inone embodiment, the ULG circuits come in various sizes, that havebetween two and eight inputs and between one and three select lines.

[0296] The extensive use of ULGs or selection circuits in this inventioncould be characterized in a number of ways in the various embodiments,those characterizations include:

[0297] (1) A digital IC design using an ASIC cell library that includesa proportion of cells with selection circuits. In various embodiments,the proportion of the cells with selection circuits in the ASIC celllibrary could include, for example, 5% or more, 10% or more, 25% ormore, 50% or more, 75% or more, 80% or more, 90% or more.

[0298] (2) A digital IC design using an ASIC cell library that includesa percentage of basic cells that each include two or more kernel cells.For example, that percentage could be more than 5%, 10%, 20%, 30%, 40%,50%, 60%, 70%, 80%, or 90%.

[0299] (2) A digital design that includes a proportion of selectioncircuits. Where the proportion is defined in terms of circuit area,power consumption or number of kernel cells. In various embodiments, theproportion of a digital design that includes selection circuits couldinclude 1% or more, 2% or more, 5% or more, 10% or more, 20% or more,30% or more, 40% or more, or 50% or more.

[0300] (3) A digital design implemented in a semiconductor circuit wherethe digital building blocks include memory cells, one input Booleanoperators and selection circuits. The selection circuits are non-Booleanoperators and have three or more inputs. In some cases, there could be asmall proportion of the digital building blocks could be Booleanoperators, such as 1% or less, 2% or less, 5% or less, 10% or less, or20% or less.

[0301] Those digital designs could be embodied in the form of, forexample, a structural netlist, a behavioral netlist, a HDL netlist, afull-custom ASIC, a semi-custom ASIC, an IP core circuit, an integratedcircuit, a hybrid chip, one or more masks, a FPGA, or a circuit cardhaving a number of integrated circuits. The fall-custom and semi-customASICs are defined as custom integrated circuits herein and could use atleast some standard cells. Structural netlists enumerate low-level cellsto use and how they are interconnected. Behavioral netlists havehigh-level descriptions of the functionality of a circuit similar tohigh-level programming languages. Case statements, IF-THEN-ELSEstatements can be easily translated from behavioral constructs to ULGcircuits. Examples of HDL netlists include RTL, VHDL, Verilog and CHDL.The HDL netlists may be high-level behavioral netlists or low-levelstructural netlists. ASIC designs can be full-custom or semi-customdesigns. The full-custom designs have a full set of masks done for eachdesign, whereas the semi-custom designs have some reusable masks thatdefine an array of gates that are custom interconnected with some uniquemasks. Where a semi-custom ASIC is done, the gates would include ULGcircuits. IP core circuits are netlists or maskwork that define areusable function such as a microprocessor, bus interface, etc. that istypically provided by a third-party vendor. An integrated circuit issimply a semiconductor chip. Where more than one chip is in a package,that package is referred to as a hybrid integrated circuit ormulti-chip-module. Circuit cards can couple together a number ofintegrated circuits soldered thereon, where the integrated circuits useULGs.

[0302] There are several approaches to integrating syntacticmanipulation into digital design. In some embodiment, the designeranticipates using syntactic manipulation at the start of the designprocess. Using HDL constructs, such as case statements, allows easymapping to the HDL ASIC cell library. Digital designers oftenmix-and-match tools from several vendors to develop a design flow suitedto a particular digital designer. With that in mind, the various belowembodiments integrate the concepts of this invention in various ways andto varying degrees. As those skilled in the art can appreciate, theprocessing of the design can be somewhat automated by using scripts torun the various tools on various design files.

[0303] With reference to FIG. 13, a block diagram of an embodiment of adesign flow 1300 that uses syntactic manipulation after synthesis isshown. Included in this design flow are an HDL entry tool 1304, asynthesis tool 1308, a conventional cell library 1324, an optimizationtool, a static & dynamic timing analysis tool 1316, and a place & routetool 1320. In this embodiment, the digital designer uses all the toolsof their normal design flow, but includes the optimization tool 1312after the synthesis tool 1308. The optimization tool 1312 performs thesyntactic manipulation in this design flow.

[0304] The HDL entry tool 1304 is a software edit tool that allows thedigital designer to enter HDL as a behavioral netlist. The HDL could beVHDL, Verilog or Concise Hardware Description Language (CHDL). CHDL is aHDL that is tuned for the design constructs beneficial for designs withULG cells. The HDL entry tool 1304 could receive feedback from the othertools to identify portions of the code that have problems found by thoseother tools. Other design capture tools could be used instead of a HDLentry tool, for example, state machine tools, RTL tools, schematiccapture tools, etc. Dynamic timing analysis could be performed on thebehavioral netlist to confirm proper functionality.

[0305] A behavioral netlist is converted by the synthesis tool 1308 intoa structural netlist using the conventional cell library 1324 in thisembodiment. The behavioral constructs are converted to Booleanconstructs and optimized. From the conventional cell library 1324, cellsare used for the optimized Boolean constructs. In some cases, typicaltiming values are used by the synthesis tool 1308 to identify potentialproblems in the conversion process using static timing analysis. Dynamictiming analysis could also be performed on the structural netlist toconfirm functionality wasn't compromised during the conversion.

[0306] The optimization tool 1312 uses syntactic manipulation to improvethe design in at least one of the following areas: power consumption,leakage current, fanout, chip area, number of masks, number of processsteps, yield, capacitance, inductance, resistance, glitches, etc. Inthis embodiment, variables can be fed to the optimization tool 1312 inorder to set the priorities among these design factors. These variablescould be set on a scale of one-to-ten to indicate relative value along asliding scale.

[0307] Some embodiments could optimize for various factors by use ofalternative cells in the ULG library. Certain cells could be optimizedfor various design factors such as power consumption, leakage current,fanout, chip area, number of masks, number of process steps, yield,capacitance, inductance, resistance, glitches, etc. During theoptimization, the alternative cells could be used based upon how thedigital designer set the priority variables.

[0308] In this embodiment, the optimization tool optimizes thestructural netlist from the synthesis tool to produce an optimizedstructural netlist that uses the same conventional cell library 1324.The synthesized structural netlist is read and converted into anintermediate netlist that uses an embodiment of the ULG ASIC celllibrary. This embodiment of the ULG cell library can be somewhatsimplified as the variation used in an ASIC could be unnecessary whenonly optimizing. Syntactic manipulation is performed upon theintermediate netlist according to the optimization priorities, ifspecified. Some embodiments could perform a default optimization thatmay or may not be modifiable by the digital designer. Once theintermediate netlist is optimized, it is converted to an optimizedstructural netlist that uses the conventional cell library 1324.

[0309] Static & dynamic timing analysis is run on the optimizedstructural netlist. The static timing analysis takes into account timingrelationships for the optimized structural netlist and identifiesportions of the circuit that may fail to meet the timing requires of thecircuit. Parameters can be entered into the synthesis and/oroptimization tools 1308, 1312 to prioritize certain portions of thecircuit to make meeting static timing requirements easier.

[0310] Dynamic timing analysis can come in a few forms. Input waveformscan be designed to stimulate the design, whereafter the digital designerchecks for correct output waveforms. To automate this process, testvectors can be developed and applied to the inputs of the circuitwhereafter output test vectors are tested against the actual output.Discrepancies are noted as errors and fixed by tweaking the behavioralcode and synthesis/optimization tools 1308, 1312 such that errors arenot introduced into the process.

[0311] Once the digital designer is happy with the structural netlist,the place & route tool 1320 performs a physical layout of the circuit. Alocation for each cell in the optimized structural netlist is chosen andtraces are laid-out to interconnect those cells according to thenetlist. These types of tools 1320 are automated or semi-automated. Moreaccurate timing values are available after place & route because thetrace lengths interconnecting the cells is known. Further static/dynamicanalysis 1316 can be performed to assure that new errors weren'tintroduced during the place and route process. Once a suitable layout isachieved, masks can be produced and fabrication can start. Dynamictesting on the resulting chip may be performed to test functionalityafter fabrication.

[0312] Referring next to FIG. 14, a block diagram of another embodimentof a design flow 1400 is shown that uses syntactic manipulation and theULG ASIC cell library. In this embodiment, a ULG cell library 1404 isused during synthesis 1308. By targeting the ULG cell library, thesynthesized structural netlist is in a format readily understood by theoptimization tool 1312 such that a conversion to an intermediate netlistis unnecessary. The optimization tool 1312 performs the syntacticmanipulation before converting from the intermediate format to theconventional cell library 1324. The optimized structural netlist is usesthe conventional cell library 1324, which is understood by the place &route tool 1320 and the fab or foundry.

[0313] With reference to FIG. 15, a block diagram of another embodimentof a design flow 1500 is shown that uses the ULG ASIC cell library 1404for the final netlist used by the fab or foundry. In this embodiment,the ULG ASIC cell library 1404 has been produced for the target processat the fab or foundry. The synthesis tool 1308 converts the behavioralnetlist into a synthesized structural netlist that uses the ULG ASICcell library 1404. The optimization tool can process the netlist withoutany conversion between cell libraries such that the resulting optimizedstructural netlist also uses the ULG ASIC cell library.

[0314] Referring next to FIG. 16, a block diagram of yet anotherembodiment of a design flow 1600 is shown that combines synthesis andsyntactic manipulation into a single tool 1604. The synthesis &optimization tool 1604 takes the behavioral netlist from the HDL entrytool 1304 and converts it to an intermediate structural netlist usingthe ULG ASIC cell library 1404. The intermediate structural netlist isoptimized using syntactic manipulation with the tool 1604 to produce anoptimized structural netlist that uses the ULG cell library 1404. Inthis embodiment, the HDL entry tool 1304 uses CHDL.

[0315] In some embodiments, the synthesis & optimization tool 1604 maynot have a ULG cell library 1404 for the target ASIC process. In thatcase, the synthesis & optimization tool 1604 would convert theintermediate netlist to an optimized netlist using the conventional celllibrary 1324.

[0316] With reference to FIG. 17, a block diagram of still anotherembodiment of a design flow 1700 is shown that uses an interactivedirect verification tool 1704 throughout the design flow 1700.Verification allows debugging a digital design throughout the designflow. In this embodiment, the interactive direct verification tool 1704tracks the evolution of the design from a behavioral netlist through toa structural netlist that has been placed and routed. The HDL entry tooluses CHDL and the design flow uses the ULG cell library 1504 for thetarget fabrication process. The various forms of the design embodied inthe evolving netlist are accessible to the interactive directverification tool 1704.

[0317] Any identified portion of the circuit can be traced through thevarious steps of the design flow 1700 using the interactive directverification tool. For example, a case statement in the CHDL behavioralcode can be followed through to the various structural netlists to seehow that case statement was implemented. Conversely, a portion of astructural netlist can be followed back to preceding structural netlistsor even the behavioral netlist.

[0318] Referring next to FIG. 18, a block diagram of still anotherembodiment of a design flow 1800 is shown that uses an interactivedirect verification tool 1804 throughout the design flow and afterfabrication. This embodiment shows the fabrication 1808 and final test1812 in the design flow. The fabrication 1808 is performed after thedesign is thoroughly tested. After production, the chips can be testedagain. Test vectors are applied to the chip inputs and scan ports in thedynamic analysis in final test 1812. Where an error can be isolated to apin or node in final test 1812, the interactive direct verification tool1804 can show the engineer the progression of the design that relates tothat failure.

[0319] With reference to FIG. 19, a flow diagram of an embodiment of adesign process 1900 is shown. In the depicted portion of the designprocess 1900, the sequential steps are show along with the test andrework steps. In step 1904, the digital design is entered using an entrytool. A HDL such as Verilog, VHDL or CHDL is used to enter thebehavioral netlist for the circuit being designed. Throughout the designentry phase, dynamic timing analysis is performed in step 1916 to verifythat the behavioral netlist is probably being prepared correctly. Atthis stage, the dynamic timing analysis is probably done in a waveformsimulation tool. Various scenarios are designed with the simulation toolto test various conditions of the digital design. During the timinganalysis of step 1916, problems could be found in step 1906. Rework ofthe behavioral code could be performed by returning to step 1904 whereproblems are found. Processing continues from step 1916 to step 1908where no problems are found.

[0320] Synthesis and optimization is performed in step 1908 to convertthe behavioral netlist to an optimized structural netlist. Conversionbetween cell libraries can also be performed in this step. Someembodiments may perform the synthesis and optimization as separatesteps. Static and dynamic timing analysis are performed in steps 1912and 1916. If problems are found, rework may be done in step 1904 or step1908. Rework in step 1908 could include changing parameters fed to thesynthesis & optimization tool 1604 or correcting problems withlibraries.

[0321] Once generally satisfied with the optimized structural netlist,processing continues to step 1920 for place & route of the design. Inthis step, the trace lengths and drive requirements are more accuratelyanalyzed to improve the verification that can be performed on thedigital design. Once again static and dynamic timing analysis isperformed in step 1912 and 1916 to verify the laid-out design stillbehaves properly. Where there are problems as detected in step 1906,rework could be performed in steps 1904, 1908 or 1920. The interactivedirect verification tool 1804 can assist the digital designer find whereflaws were likely introduced into the netlist. Where there are noproblems after layout, the design is fabricated in step 1924.

[0322] Having described several embodiments, it will be recognized bythose of skill in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. Accordingly, the above description should notbe taken as limiting the scope of the invention, which is defined in thefollowing claims.

What is claimed is:
 1. An ASIC cell library for use in creation ofcustom integrated circuits, the ASIC cell library comprising: aplurality of first cells; a plurality of second cells, wherein: each ofthe plurality of second cells are comprised of two or more kernel cells,and the ASIC cell library is at least 5% comprised of second cells. 2.The ASIC cell library for use in creation of custom integrated circuitsof claim 1, wherein the ASIC cell library is one of 10% or more, 20% ormore, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more,80% or more, 90% or more, and 95% or more comprised of second cells. 3.The ASIC cell library for use in creation of custom integrated circuitsof claim 1, wherein the plurality of first cells includes no kernelcells.
 4. The ASIC cell library for use in creation of custom integratedcircuits of claim 1, wherein each of the plurality of first cells is notentirely composed of kernel cells.
 5. The ASIC cell library for use increation of custom integrated circuits of claim 1, wherein a totalnumber of kernel cells is less than one of 10, 20, 30, 40, 50, 75, and100.
 6. The ASIC cell library for use in creation of custom integratedcircuits of claim 1, wherein each of the plurality of second cells hasat least two of a selection circuit, a memory circuit and an one-inputBoolean circuit.
 7. The ASIC cell library for use in creation of customintegrated circuits of claim 1, wherein each of the plurality of secondcells has at least two of a selection circuit, a memory circuit and abuffer circuit.
 8. The ASIC cell library for use in creation of customintegrated circuits of claim 7, wherein each selection circuit is chosenfrom a group consisting of a 2 to 1 multiplexor, a 4 to 1 multiplexorand a 8 to 1 multiplexor.
 9. The ASIC cell library for use in creationof custom integrated circuits of claim 7, wherein each memory circuit ischosen from a group consisting of a D flip-flop, a setable D flip-flop,a resetable D flip-flop, a J-K flip-flop, and a flip-flop with scancapability.
 10. The ASIC cell library for use in creation of customintegrated circuits of claim 7, where each memory circuit is chosen froma group consisting of an asynchronous triggered flip-flop, a rising-edgesynchronous triggered flip-flop, a falling-edge synchronous triggeredflip-flop, a synchronous triggered flip-flop, and an edge triggeredflip-flop.
 11. The ASIC cell library for use in creation of customintegrated circuits of claim 7, wherein the buffer circuit is chosenfrom a group consisting of a non-inverting buffer, an inverting buffer,a high-drive buffer, and a tristatable buffer.
 12. The ASIC cell libraryfor use in creation of custom integrated circuits of claim 7, whereinthe buffer circuit is chosen from a group consisting of a non-invertingbuffer, an inverting buffer, a high-drive buffer, a tristatable buffer,and an inverting buffer with an enable input.
 13. The ASIC cell libraryfor use in creation of custom integrated circuits of claim 7, whereinthe selection circuit passes one of a plurality of inputs to its output.14. The ASIC cell library for use in creation of custom integratedcircuits of claim 1, wherein substantially all of the kernel cells haveat least some hand-layout.
 15. The ASIC cell library for use in creationof custom integrated circuits of claim 1, wherein the kernel cells havea substantially uniform height.
 16. The ASIC cell library for use increation of custom integrated circuits of claim 1, wherein at least oneof the plurality of second cells is comprised of two or more kernelcells connected using abutment.
 17. The ASIC cell library for use increation of custom integrated circuits of claim 1, wherein kernel cellsare connected by abutment to form the plurality of second cells.
 18. TheASIC cell library for use in creation of custom integrated circuits ofclaim 1, wherein abutment aligns a first I/O of a first kernel cell witha second I/O of a second kernel cell along a latitude across anintegrated circuit.
 19. The ASIC cell library for use in creation ofcustom integrated circuits of claim 1, wherein: the kernel cells includea plurality of different selection circuit types, memory circuit typesand buffer circuit types, the selection circuit types include an output,the memory circuit types include a memory input, the buffer circuittypes include a buffer input, and the output is aligned latitudinallywith at least one of the memory input and the buffer input across a rowof kernel cells on an integrated circuit.
 20. The ASIC cell library foruse in creation of custom integrated circuits of claim 1, wherein: thekernel cells include a plurality of different selection circuit types,memory circuit types and buffer circuit types, the memory circuit typesinclude a memory output, the buffer circuit types include a bufferinput, and the memory output is aligned latitudinally with the bufferinput across a row of kernel cells on an integrated circuit.
 21. TheASIC cell library for use in creation of custom integrated circuits ofclaim 1, wherein: the kernel cells include a plurality of differentmemory circuit types, and the memory circuit types with a clock inputalign the clock input latitudinally across a row of kernel cells on anintegrated circuit.
 22. An integrated circuit cell library for use incircuits having digital logic, the integrated circuit comprising: aplurality of first cells that is a first percentage of the integratedcircuit cell library; a plurality of second cells that is a secondpercentage of the integrated circuit cell library, wherein: each of theplurality of second cells is comprised of a selection circuit, thesecond percentage is more than 5% of the integrated circuit celllibrary.
 23. The integrated circuit cell library for use in circuitshaving digital logic as recited in claim 22, wherein the secondpercentage is chosen from the group consisting of 10% or more, 20% ormore, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more,80% or more, 90% or more, and 95% or more.
 24. The integrated circuitcell library for use in circuits having digital logic as recited inclaim 22, wherein the kernel cells have a substantially uniform height.25. The integrated circuit cell library for use in circuits havingdigital logic as recited in claim 22, wherein at least one of theplurality of second cells is comprised of two or more kernel cellsconnected using abutment.
 26. The integrated circuit cell library foruse in circuits having digital logic as recited in claim 22, whereinkernel cells are connected by abutment to form the plurality of secondcells.
 27. The integrated circuit cell library for use in circuitshaving digital logic as recited in claim 22, wherein abutment aligns afirst I/O of a first kernel cell with a second I/O of a second kernelcell along a latitude across an integrated circuit.
 28. An integratedcircuit cell library for use in circuits having digital logic, theintegrated circuit comprising: a plurality of first cells that is afirst percentage of the integrated circuit cell library; a plurality ofsecond cells that is a second percentage of the integrated circuit celllibrary, a plurality of kernel cells that are used in forming theplurality of second cells, wherein at least some of the plurality ofsecond cells include at least two kernel cells coupled together throughabutment.
 29. The integrated circuit cell library for use in circuitshaving digital logic as recited in claim 28, wherein the kernel cellshave a substantially uniform height.
 30. The integrated circuit celllibrary for use in circuits having digital logic as recited in claim 28,wherein abutment aligns a first I/O of a first kernel cell with a secondI/O of a second kernel cell along a latitude across an integratedcircuit.